April’s talk

Professor Martin Hardcastle of the University of Hertfordshire is one of those people we need to invite back in the future. His talk was about surveying the sky for radio galaxies. In 1933, Karl Jansky’s first radio survey showed the Milky Way and the Sun and that’s all. Since then, we know that galaxies have a wide range of radio profiles.

The first major radio surveys were done at the Cambridge Lord’s Bridge site. The old radio array (do they still call them Yagi?) is where the 3rd and 4th Cambridge Catalogue of radio sources was compiled (the 1st and 2nd were full of spurious readings). You will have heard of the Parkes radio source catalogue, from the radio observatory in Australia. There is also the B2 catalogue of around 10,000 sources from 1960 to 1975. Please google for further information.

Some of the sources were very bright, but visually very dim, so the 3rd Catalogue’s sources took 30 years to record at visual wavelengths; he mentioned one 19.5 magnitude galaxy seen between two bright radio lobes. These lobes had emitted from the nucleus of the galaxy. Now Prof Hardcastle started warming to his task!

It seems all reasonably large galaxies have a black hole in the centre, caused when orbiting stuff gravitates towards the centre over time. And when stuff starts building up gravitationally around it you also get a build up of magnetic fields. And, as it’s rotating, so the particles in the stuff get twisted around. He showed us a computer simulation on auto-replay showing how there is a massive flash then two blobs appear in the polar regions, which then become the lobes over 100million years. Mesmerising stuff. Depending on circumstances there may already be a halo of debris around the galaxy or the jets just spread out into the intergalactic medium. The strong magnetic fields are what makes some of these jets reach relativistic speeds near to the central black hole, i.e., half of the speed of light or even faster. The big elliptical galaxies are most exciting. My notes on M87 say it is an active old mess in the visual but far more active in radio wavelengths and has a huge radio envelope.

As a result of such activity you can imagine there is a wide frequency range needed to monitor all this radio output.

The Chandra (an orbiting x-ray telescope) website shows lots of x-ray and radio sources, and reveals that the envelope is hot gas interacting with the ‘intercluster medium’. Google Cygnus A.

The VLA (Very Large Array = 27 radio dishes which can be laid out in four different layouts for different resolutions) has a range of 75MHz (so a bit shorter wavelength than Radio Oxford at 95.2 MHz) down to 43 GHz. When working to GHz wavelengths its resolution is sub arc seconds. It does have a small field of view, however. It has recorded 100,000 sources and counting, from 1995 onwards. Its radio survey is called FIRST (Faint Images of the Radio Sky at Twenty cm) is a shorter wavelength study (1 to 4GHz). It has 5 arcsec resolution but again only has a small field of view.

More recently and increasing in size and resolution is the LOFAR (LOw Frequency Array) with a range of 30 to 300MHz (Anything less than 30Mhz is absorbed by the ionosphere). This area is mostly unexplored and FM radio sits in there too. These long wavelengths need very long base lines, so they are observed by lots of little antennae, like poles with strings attached. They are camped in fields in large units called Superterps. The HQ is in Groningen, Netherlands, but there are five stations in Germany, with others in France, Sweden, Poland and Ireland. We have one in Chilbolton, Hampshire. The data is coordinated via VLBI (Very Long Base-line Interferometry).

LOFAR is dishing out masses of data, and this is accessible to you (yes, you) via the galaxy zoo website for citizen science.

Another new kid on the block is the SKA, (Square Kilometre Array), which also has pole antennae but also dishes for shorter wavelengths to 14GHz. There are stations in South Africa and Western Australia. I googled SKA and on April 28th the Global HQ is now being built in the grounds of Jodrell Bank.

This data, along with data from Space telescope Herschel, is now creating quite a map of what is in the sky, and is able to separate the active galactic nuclei from those that are star forming, as a result of core activity in the past. Prof Hardcastle hopes we will end up with a Hertsprung-Russell diagram in the radio, with size versus temperature, just as there is with stars.

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